The present invention relates to a furnace discharge system and method of operation for a rotary hearth furnace. More particularly, the present invention relates to a furnace discharge system for a rotary hearth furnace including at least two helical augers that cooperatively transition material from a continuous process within the rotary hearth furnace to a non-continuous process exterior of the rotary hearth furnace.
Briefly, a rotary hearth furnace (RHF) is a continuous reheating furnace generally having an annular inner wall circumscribed by a spaced annular outer wall. The space between the inner wall and the outer wall includes a circular rotating hearth. Burners may be installed in the inner and outer walls and in the roof. Combustion and process effluent gases are permitted to vent through a flue located in the roof or in a side wall.
Rotary hearth furnaces have been typically used for heating metallic sections for downstream mechanical forming; i.e., forging or rolling. A further application is found for heating solid material with an internal gaseous furnace atmosphere suitable for prevention of metal oxidation and/or reduction of metal oxide present in the solid material. The rotary hearth furnace is equipped with inner and outer water seals which makes it ideal for containment of the internal gases required for high temperature oxidation resistance and/or reduction.
Generally material, such as pellets, briquettes loose mixtures and agglomerates is uniformly distributed and loaded (dropped) onto the rotating hearth by a conveyor or chute for processing within the furnace. After the material is conveyed along the hearth path and processed within the furnace, the material may be in the form of pellets, briquettes, loose mixtures, agglomerates or a fused mass, and is discharged from the furnace using a single material discharge auger. The discharge auger typically consists of a central shaft with solid helical metal flights welded or bolted thereto projecting outwardly from the central shaft. The discharge auger extends across the width of the circular hearth and is connected to an external motor for rotation. The discharge auger continuously conveys the material from the hearth down a refractory chute and into a transport canister.
In applications where the material from the RHF is used downstream in a non-continuous batch process, the single auger arrangement requires an elaborate design starting with a discharge chute, discharge chute valving and/or multiple canisters and arc gate (dump valve) to control delivery of the material into a transport canister. It will be appreciated that the canisters for receiving the material from the furnace must be arranged on a turntable and indexed into and out of the load positions beneath the canister and arc gate. During this indexing, the continuous discharge of material is delivered to the canister while the filled canister is being indexed to make room for a fresh transport canister.
Alternatively, the single discharge auger can be arranged to deliver the hot product into a bifurcated chute suitable for alternatively discharging into multiple transport canisters. This arrangement requires a two-way diversion valve or arc-gate for directing the flow of material to the empty transport canister. Each leg of the of the bifurcated chute requires that a gate valve be located at the bottom terminus. It will be appreciated that considerable height is necessary to accommodate the bifurcated chute arrangement.
This single material discharge auger arrangement, in addition to equipment complexity, can contribute to operational problems and unwanted down time due to potential bridging of material in the hopper and/or failure of the dump valve (arc gate) to open or close properly. It will be appreciated that closure of the gate can be impeded by material interfering with the full swing of the gate. It will be further appreciated that the incomplete closure of the gate valve may permit extremely hot material to continuously discharge from the furnace and permit air (oxygen) to be in-drafted into the furnace thereby providing extremely hazardous conditions for auto combustion when a highly reducing atmosphere (CO and H2) is present in the RHF.
The present invention relates to an improved RHF discharge system that addresses the problems associated with a single material auger discharge. The present invention also facilitates the transition from the continuous process of the RHF to a non-continuous process downstream of the RHF. The dual material auger arrangement of the present invention enables the material transport canister transfer to proceed without the need for an intermediate retention hopper and material supporting arc gate. The simplicity of the discharge assembly of the present invention also simplifies the RHF process plant arrangements.
Briefly, the present invention relates to a furnace discharge system for removing material from a hearth of a rotary hearth furnace. The system includes at least two material discharge augers positioned above the hearth of the rotary hearth furnace. Each discharge auger is operatively mounted to allow for height adjustment of the discharge auger in alternating sequence from a material removal position to an elevated material by-pass position.
The material is discharged from the rotary hearth furnace by lowering at least one discharge auger to a material removal position and elevating at least one discharge auger to an elevated material by-pass position. A material transport canister is operatively attached to the discharge chute and the valve of the discharge chute of the elevated discharge auger is closed and the valve of the lowered discharge auger is opened. Material from the rotary hearth furnace is then conveyed into the material transport canister.